Properties of walnut influenced by short time microwave treatment for disinfestation of insect infestation

Insect infestation is the major cause of losses in many stored products such as grains, seeds, cereals and nuts. The use of chemicals to control insect infestation has widely been adopted but now is a cause of concern because of hazardous side effects. Alternative quarantine methods such as ionizing radiation, controlled atmosphere, conventional hot air treatment and dielectric heating have been suggested. Recent studies have indicated that microwave treatment is a potential means of replacing other techniques because of selective heating, absence of pollution in the environment, the achieving of equivalent or better quality retention and energy minimization. However improper application of microwaves can cause irreversible changes in the quality of the final product. The main objective of this research is to evaluate the effect of microwave power level and exposure time on the quality of walnuts. The quality parameters studied were water activity, colour change, temperature rise, peroxide value (PV) and free fatty acid levels (FFA). It was found that microwave power level and exposure time significantly affected colour change and temperature rise. Untreated walnut kernels exhibited significantly higher (p < 0.05) peroxide and FFA values than treated walnut kernels. Microwave treatment to target temperatures of 50-55 °C (which is unfavourable for insect survival) made the PV of walnut drop down to between 1.35 and 1.42 meqO2/Kg (from the initial value of 2.89 ± 0.048) and FFA value to 0.63-0.69% (from 1.08 ± 0.037).